Hitherto, as a control system having a limiting function, a control system A shown in FIG. 12 has been known. An output (control variable) y of a controlled object B is controlled by the control system A.
The control system A is equipped with a manipulation variable determining unit A1, a limiter A2, and an integrating unit A3.
The manipulation variable determining unit A1 has a transmission element G0 and outputs a primary manipulation variable x1 according to an expression (a) shown below on the basis of a difference y0−y between a desired value y0 and the control variable y.x1=G0·(y0−y)  (a)
The limiter A2 outputs a secondary manipulation variable x2 on the basis of an input of the primary manipulation variable x1. To be more specific, the limiter A2 directly outputs the primary manipulation variable x1 as the secondary manipulation variable x2 if the primary manipulation variable x1 remains in a predetermined range. If, on the other hand, the primary manipulation variable x1 is out of the predetermined range, then the limiter A2 outputs a boundary value of the range as the secondary manipulation variable x2.
The integrating unit A3 has an integrating element K/s and outputs a final manipulation variable x according to an expression (b) given below on the basis of an input of a difference between the second manipulation variable x2 and the control variable y.x=(K/s)·(x2−y)  (b)
The controlled object B has a transmission element G and outputs the control variable y according to an expression (c) given below on the basis of an input of the manipulation variable x.y=G·x+d  (c)
where “d” denotes a steady-state deviation (offset) in the controlled object B.
According to the control system A having the aforesaid construction, if the control variable y is likely to be out of an “allowable range” based on the primary manipulation variable x1, then the secondary manipulation variable x2 is determined such that the primary manipulation variable x1 is limited to be within the predetermined range by the limiter A2. Thus, the control is conducted so as to restrict the control variable y to be within the allowable range.
A feedback loop comprised of the integrating unit A3 and an adder A4 located immediately upstream relative to the integrating unit A3 determines the control variable x such that the steady-state deviation d of the controlled object B is cancelled. This allows control to be carried out such that the control variable y remains in the allowable range even if there is the steady-state deviation d.
If, however, a gain coefficient K (refer to expression (b)) in the integrating unit A3 is high, then the feedback loop becomes unstable and oscillates or vibrates; therefore, the gain coefficient K must be controlled low. Thus, the responsiveness of the control variable y output from the controlled object B relative to the desired value y0 input to the control system A undesirably deteriorates. As a result, an error (inaccuracy) of the control variable y relative to the desired value y0 increases, and control accuracy may deteriorate and the control variable y may be out of the allowable range.
Accordingly, the present invention is intended to provide, as a solution, a control system that permits stable and accurate control so as to maintain a control variable in an allowable range.